[patch 1/3] Trace code and documentation

David J. Wilder
2007-09-26T18:23:10Z

Trace - Provides tracing primitives

Signed-off-by: Tom Zanussi <zanussi@us.ibm.com>
Signed-off-by: Martin Hunt <hunt@redhat.com>
Signed-off-by: David Wilder <dwilder@us.ibm.com>
---
Documentation/trace.txt | 160 ++++++++++++++
include/linux/trace.h | 99 +++++++++
lib/Kconfig | 9 +
lib/Makefile | 2 +
lib/trace.c | 563 +++++++++++++++++++++++++++++++++++++++++++++++
5 files changed, 833 insertions(+), 0 deletions(-)

diff --git a/Documentation/trace.txt b/Documentation/trace.txt
new file mode 100644
index 0000000..0e42fb8
--- /dev/null
+++ b/Documentation/trace.txt
@@ -0,0 +1,160 @@
+Trace Setup and Control
+=======================
+In the kernel, the trace interface provides a simple mechanism for
+starting and managing data channels (traces) to user space. The
+trace interface builds on the relay interface. For a complete
+description of the relay interface, please see:
+Documentation/filesystems/relay.txt.
+
+The trace interface provides a single layer in a complete tracing
+application. Trace provides a kernel API that can be used for the setup
+and control of tracing channels. User of trace must provide a data layer
+responsible for formatting and writing data into the trace channels.
+
+A layered approach to tracing
+=============================
+A complete kernel tracing application consists of a data provider and
+a data consumer. Both provider and consumer contain three layers; each
+layer works in tandem with the corresponding layer in the opposite side.
+The layers are represented in the following diagram.
+
+Provider Data layer
+ Formats raw trace data and provides data-related service.
+ For example, adding timestamps used by consumer to sort data.
+
+Provider Control layer
+ Provided by the trace interface, this layer creates trace channels
+ and informs the data layer and consumer of the current state
+ of the trace channels.
+
+Provider Buffering layer
+ Provided by relay. This layer buffers data in the
+ kernel for consumption by the consumer's buffer
+ layer.
+
+Provider (in-kernel facility)
+-----------------------------------------------------------------------------
+Consumer (user application)
+
+
+Consumer Buffer layer
+ Reads/consumes data from the provider's data buffers.
+
+Consumer Control layer
+ Communicates to the provider's control layer to control the state
+ of the trace channels.
+
+Consumer Data layer
+ Sorts and formats data as provided by the provider's data layer.
+
+The provider is coded as a kernel facility. The consumer is coded as
+a user application.
+
+
+Trace - Features
+================
+Trace exploits services and features provided by relay. These features
+are:
+- The creation and destruction of relay channels.
+- Buffer management. Overwrite or non-overwrite modes can be selected
+ as well as global or per-CPU buffering.
+
+Overwrite mode can be called "flight recorder mode". Flight recorder
+mode is selected by setting the TRACE_FLIGHT_CHANNEL flag when
+creating trace channels. In flight mode when a tracing buffer is
+full, the oldest records in the buffer will be discarded to make room
+as new records arrive. In the default non-overwrite mode, new records
+may be written only if the buffer has room. In either case, to
+prevent data loss, a user space reader must keep the buffers
+drained. Trace provides a means to detect the number of records that
+have been dropped due to a buffer-full condition (non-overwrite mode
+only).
+
+When per-CPU buffers are used, relay creates one debugfs file for each
+running CPU. The user-space consumer of the data is responsible for
+reading the per-CPU buffers and collating the records presumably using
+a time stamp or sequence number included in the trace records. The
+use of global buffers eliminates this extra work of sequencing
+records; however the provider's data layer must hold a lock when
+writing records. The lock prevents writers running on different CPUs
+from overwriting each other's data. However, buffering may be slower
+because writes to the buffer are serialized. Global buffering is
+selected by setting the TRACE_GLOBAL_CHANNEL flag when creating trace
+channels.
+
+Trace User Interface
+===================
+When a trace channel is created and started, the following
+directories and files are created in the root of the mounted debugfs.
+
+/debug (root of the debugfs)
+ /<trace-root-dir>
+ /<trace-name>
+ trace0...traceN-1 Per-CPU trace data, one
+ file per CPU.
+
+ state Start or stop tracing by
+ by writing the strings
+ "start" or "stop" to this
+ file. Read the file to get the
+ current state.
+
+ dropped The number of records dropped
+ due to a full-buffer condition,
+ for non-TRACE_FLIGHT_CHANNELs
+ only.
+
+ rewind Trigger a rewind by writing
+ to this file. i.e. start
+ next read at the beginning
+ again. Only available for
+ TRACE_FLIGHT_CHANNELS.
+
+
+ nr_sub Number of sub-buffers
+ in the channel.
+
+ sub_size Size of sub-buffers in
+ the channnel.
+
+Trace data is gathered from the trace[0...N] files using one of the
+available interfaces provided by relay.
+
+When using the read(2) interface, as data is read it is marked as
+consumed by the relay subsystem. Therefore, subsequent reads will
+only return unconsumed data.
+
+Trace Kernel API
+===============
+An overview of the trace Kernel API is now given. More details of the
+API can be found in linux/trace.h.
+
+The steps a kernel data provider takes to utilize the trace interface are:
+1) Set up a trace channel - trace_setup()
+2) Start the trace channel - trace_start()
+3) Write one or more trace records into the channel (using the relay API).
+
+ Important: When writing a trace record the provider must insure that
+ preemption is disabled and that trace state is set to "running". A
+ typical function used to write records into a trace channel should
+ follow the following semantics:
+
+ rcu_read_lock(); // disables preemption
+ if (trace_running(trace)){
+ relay_write(....); // use any available relay data
+ // function
+ }
+ rcu_read_unlock(); // enables preemption
+
+4) Stop and start tracing as desired - trace_start()/trace_stop()
+5) Destroy the trace channel and underlying relay channel -
+ trace_cleanup().
+
+Credits
+=======
+Trace is adapted from blktrace authored by Jens Axboe (axboe@kernel.dk).
+
+Major contributions were made by:
+Tom Zanussi <zanussi@us.ibm.com>
+Martin Hunt <hunt@redhat.com>
+David Wilder <dwilder@us.ibm.com>
diff --git a/include/linux/trace.h b/include/linux/trace.h
new file mode 100644
index 0000000..a3a10c9
--- /dev/null
+++ b/include/linux/trace.h
@@ -0,0 +1,99 @@
+/*
+ * TRACE defines and function prototypes
+ *
+ * Copyright (C) 2007 IBM Inc.
+ *
+ * Tom Zanussi <zanussi@us.ibm.com>
+ * Martin Hunt <hunt@redhat.com>
+ * David Wilder <dwilder@us.ibm.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ *
+ */
+#ifndef _LINUX_TRACE_H
+#define _LINUX_TRACE_H
+
+#include <linux/relay.h>
+
+/*
+ * TRACE channel flags
+ */
+#define TRACE_GLOBAL_CHANNEL 0x01
+#define TRACE_FLIGHT_CHANNEL 0x02
+#define TRACE_DISABLE_STATE 0x04
+
+enum trace_state {
+ TRACE_SETUP,
+ TRACE_RUNNING,
+ TRACE_STOPPED,
+};
+
+#define TRACE_ROOT_NAME_SIZE 64 /* Max root dir identifier */
+#define TRACE_NAME_SIZE 64 /* Max trace identifier */
+
+/*
+ * Global root user information
+ */
+struct trace_root {
+ struct list_head list;
+ char name[TRACE_ROOT_NAME_SIZE];
+ struct dentry *root;
+ unsigned int users;
+};
+
+/*
+ * Client information
+ */
+struct trace_info {
+ struct mutex state_mutex; /* Used to protect state changes */
+ enum trace_state state;
+ struct dentry *state_file;
+ struct rchan *rchan;
+ struct dentry *dir;
+ struct dentry *dropped_file;
+ struct dentry *reset_consumed_file;
+ struct dentry *nr_sub_file;
+ struct dentry *sub_size_file;
+ atomic_t dropped;
+ struct trace_root *root;
+ void *private_data;
+ unsigned int flags;
+ unsigned int buf_size;
+ unsigned int buf_nr;
+};
+
+#ifdef CONFIG_TRACE
+static inline int trace_running(struct trace_info *trace)
+{
+ return trace->state == TRACE_RUNNING;
+}
+struct trace_info *trace_setup(const char *root, const char *name,
+ u32 buf_size, u32 buf_nr, u32 flags);
+int trace_start(struct trace_info *trace);
+int trace_stop(struct trace_info *trace);
+void trace_cleanup(struct trace_info *trace);
+#else
+static inline struct trace_info *trace_setup(const char *root,
+ const char *name, u32 buf_size,
+ u32 buf_nr, u32 flags)
+{
+ return NULL;
+}
+static inline int trace_start(struct trace_info *trace) { return -EINVAL; }
+static inline int trace_stop(struct trace_info *trace) { return -EINVAL; }
+static inline int trace_running(struct trace_info *trace) { return 0; }
+static inline void trace_cleanup(struct trace_info *trace) {}
+#endif
+
+#endif
diff --git a/lib/Kconfig b/lib/Kconfig
index ba3d104..ad19f87 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -141,4 +141,13 @@ config HAS_DMA
config CHECK_SIGNATURE
bool

+config TRACE
+ bool "Trace setup and control"
+ depends on RELAY && DEBUG_FS
+ help
+ This option provides support for the setup, teardown and control
+ of tracing channels from kernel code. It also provides trace
+ information and control to userspace via a set of debugfs control
+ files. If unsure, say N.
+
endmenu
diff --git a/lib/Makefile b/lib/Makefile
index 76d6619..fa00ea1 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -70,6 +70,8 @@ lib-$(CONFIG_GENERIC_BUG) += bug.o

obj-$(CONFIG_PROFILE_LIKELY) += likely_prof.o

+obj-$(CONFIG_TRACE) += trace.o
+
hostprogs-y := gen_crc32table
clean-files := crc32table.h

diff --git a/lib/trace.c b/lib/trace.c
new file mode 100644
index 0000000..a760597
--- /dev/null
+++ b/lib/trace.c
@@ -0,0 +1,563 @@
+/*
+ * Based on blktrace code, Copyright (C) 2006 Jens Axboe <axboe@kernel.dk>
+ * Moved to utt.c by Tom Zanussi <zanussi@us.ibm.com>, 2006
+ * Additional contributions by:
+ * Martin Hunt <hunt@redhat.com>, 2007
+ * David Wilder <dwilder@us.ibm.com>, 2007
+ * Renamed to trace <dwilder.ibm.com>, 2007
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ *
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/mutex.h>
+#include <linux/debugfs.h>
+#include <linux/trace.h>
+
+static LIST_HEAD(trace_roots);
+static DEFINE_MUTEX(trace_mutex);
+
+static int state_open(struct inode *inode, struct file *filp)
+{
+ filp->private_data = inode->i_private;
+ return 0;
+}
+
+static ssize_t state_read(struct file *filp, char __user *buffer,
+ size_t count, loff_t *ppos)
+{
+ struct trace_info *trace = filp->private_data;
+ char *buf = "trace not started\n";
+
+ if (trace->state == TRACE_STOPPED)
+ buf = "stopped\n";
+ else if (trace->state == TRACE_RUNNING)
+ buf = "running\n";
+ return simple_read_from_buffer(buffer, count, ppos, buf, strlen(buf));
+}
+
+static ssize_t state_write(struct file *filp, const char __user *buffer,
+ size_t count, loff_t *ppos)
+{
+ struct trace_info *trace = filp->private_data;
+ char buf[16];
+ int ret;
+
+ if (trace->flags & TRACE_DISABLE_STATE)
+ return -EINVAL;
+
+ if (count > sizeof(buf) - 1)
+ return -EINVAL;
+
+ if (copy_from_user(buf, buffer, count))
+ return -EFAULT;
+
+ buf[count] = '\0';
+
+ if (strncmp(buf, "start", strlen("start")) == 0) {
+ ret = trace_start(trace);
+ if (ret)
+ return ret;
+ } else if (strncmp(buf, "stop", strlen("stop")) == 0)
+ trace_stop(trace);
+ else
+ return -EINVAL;
+
+ return count;
+}
+
+static struct file_operations state_fops = {
+ .owner = THIS_MODULE,
+ .open = state_open,
+ .read = state_read,
+ .write = state_write,
+};
+
+static void remove_root(struct trace_info *trace)
+{
+ if (trace->root->root && simple_empty(trace->root->root)) {
+ debugfs_remove(trace->root->root);
+ list_del(&trace->root->list);
+ kfree(trace->root);
+ trace->root = NULL;
+ }
+}
+
+static void remove_tree(struct trace_info *trace)
+{
+ mutex_lock(&trace_mutex);
+ debugfs_remove(trace->dir);
+
+ if (trace->root) {
+ if (--trace->root->users == 0)
+ remove_root(trace);
+ }
+
+ mutex_unlock(&trace_mutex);
+}
+
+/*
+ * Creates the trace_root if it's not found.
+ */
+static struct trace_root *lookup_root(const char *root)
+{
+ struct list_head *pos;
+ struct trace_root *r;
+
+ list_for_each(pos, &trace_roots) {
+ r = list_entry(pos, struct trace_root, list);
+ if (!strcmp(r->name, root))
+ return r;
+ }
+
+ r = kzalloc(sizeof(struct trace_root), GFP_KERNEL);
+ if (!r)
+ return ERR_PTR(-ENOMEM);
+
+ strlcpy(r->name, root, sizeof(r->name));
+
+ r->root = debugfs_create_dir(root, NULL);
+ if (IS_ERR(r->root))
+ r->root = NULL;
+ else
+ list_add(&r->list, &trace_roots);
+
+ return r;
+}
+
+static struct dentry *create_tree(struct trace_info *trace, const char *root,
+ const char *name)
+{
+ struct dentry *dir = NULL;
+
+ if (root == NULL || name == NULL)
+ return ERR_PTR(-EINVAL);
+
+ mutex_lock(&trace_mutex);
+
+ trace->root = lookup_root(root);
+ if (IS_ERR(trace->root)) {
+ trace->root = NULL;
+ goto err;
+ }
+
+ dir = debugfs_create_dir(name, trace->root->root);
+ if (IS_ERR(dir))
+ remove_root(trace);
+ else
+ trace->root->users++;
+
+err:
+ mutex_unlock(&trace_mutex);
+ return dir;
+}
+
+static int dropped_open(struct inode *inode, struct file *filp)
+{
+ filp->private_data = inode->i_private;
+
+ return 0;
+}
+
+static ssize_t dropped_read(struct file *filp, char __user *buffer,
+ size_t count, loff_t *ppos)
+{
+ struct trace_info *trace = filp->private_data;
+ char buf[16];
+
+ snprintf(buf, sizeof(buf), "%u\n", atomic_read(&trace->dropped));
+
+ return simple_read_from_buffer(buffer, count, ppos, buf, strlen(buf));
+}
+
+static struct file_operations dropped_fops = {
+ .owner = THIS_MODULE,
+ .open = dropped_open,
+ .read = dropped_read,
+};
+
+static int reset_consumed_open(struct inode *inode, struct file *filp)
+{
+ filp->private_data = inode->i_private;
+
+ return 0;
+}
+
+static ssize_t reset_consumed_write(struct file *filp,
+ const char __user *buffer,
+ size_t count, loff_t *ppos)
+{
+ int ret = count;
+ struct trace_info *trace = filp->private_data;
+
+ mutex_lock(&trace->state_mutex);
+ switch (trace->state) {
+ case TRACE_RUNNING:
+ trace->state = TRACE_STOPPED;
+ synchronize_rcu();
+ relay_flush(trace->rchan);
+ relay_reset_consumed(trace->rchan);
+ trace->state = TRACE_RUNNING;
+ break;
+ case TRACE_STOPPED:
+ relay_reset_consumed(trace->rchan);
+ break;
+ default:
+ ret = -EINVAL;
+ }
+ mutex_unlock(&trace->state_mutex);
+ return ret;
+}
+
+static struct file_operations reset_consumed_fops = {
+ .owner = THIS_MODULE,
+ .open = reset_consumed_open,
+ .write = reset_consumed_write
+};
+
+static int sub_size_open(struct inode *inode, struct file *filp)
+{
+ filp->private_data = inode->i_private;
+
+ return 0;
+}
+
+static ssize_t sub_size_read(struct file *filp, char __user *buffer,
+ size_t count, loff_t *ppos)
+{
+ struct trace_info *trace = filp->private_data;
+ char buf[32];
+
+ snprintf(buf, sizeof(buf), "%zu\n", trace->rchan->subbuf_size);
+
+ return simple_read_from_buffer(buffer, count, ppos, buf, strlen(buf));
+}
+
+static struct file_operations sub_size_fops = {
+ .owner = THIS_MODULE,
+ .open = sub_size_open,
+ .read = sub_size_read,
+};
+
+static int nr_sub_open(struct inode *inode, struct file *filp)
+{
+ filp->private_data = inode->i_private;
+ return 0;
+}
+
+static ssize_t nr_sub_read(struct file *filp, char __user *buffer,
+ size_t count, loff_t *ppos)
+{
+ struct trace_info *trace = filp->private_data;
+ char buf[32];
+
+ snprintf(buf, sizeof(buf), "%zu\n", trace->rchan->n_subbufs);
+
+ return simple_read_from_buffer(buffer, count, ppos, buf, strlen(buf));
+}
+
+static struct file_operations nr_sub_fops = {
+ .owner = THIS_MODULE,
+ .open = nr_sub_open,
+ .read = nr_sub_read,
+};
+
+/*
+ * Keep track of how many times we encountered a full subbuffer, to aid
+ * the user space app in telling how many lost events there were.
+ */
+static int subbuf_start_callback(struct rchan_buf *buf, void *subbuf,
+ void *prev_subbuf, size_t prev_padding)
+{
+ struct trace_info *trace = buf->chan->private_data;
+
+ if (trace->flags & TRACE_FLIGHT_CHANNEL)
+ return 1;
+
+ if (!relay_buf_full(buf))
+ return 1;
+
+ atomic_inc(&trace->dropped);
+
+ return 0;
+}
+
+static int remove_buf_file_callback(struct dentry *dentry)
+{
+ debugfs_remove(dentry);
+
+ return 0;
+}
+
+static struct dentry *create_buf_file_callback(const char *filename,
+ struct dentry *parent, int mode,
+ struct rchan_buf *buf,
+ int *is_global)
+{
+ return debugfs_create_file(filename, mode, parent, buf,
+ &relay_file_operations);
+}
+
+static struct dentry *create_global_buf_file_callback(const char *filename,
+ struct dentry *parent,
+ int mode,
+ struct rchan_buf *buf,
+ int *is_global)
+{
+ *is_global = 1;
+
+ return debugfs_create_file(filename, mode, parent, buf,
+ &relay_file_operations);
+}
+
+static struct rchan_callbacks relay_callbacks = {
+ .subbuf_start = subbuf_start_callback,
+ .create_buf_file = create_buf_file_callback,
+ .remove_buf_file = remove_buf_file_callback,
+};
+static struct rchan_callbacks relay_callbacks_global = {
+ .subbuf_start = subbuf_start_callback,
+ .create_buf_file = create_global_buf_file_callback,
+ .remove_buf_file = remove_buf_file_callback,
+};
+
+static void remove_controls(struct trace_info *trace)
+{
+ debugfs_remove(trace->state_file);
+ debugfs_remove(trace->dropped_file);
+ debugfs_remove(trace->reset_consumed_file);
+ debugfs_remove(trace->nr_sub_file);
+ debugfs_remove(trace->sub_size_file);
+ remove_tree(trace);
+}
+
+/*
+ * Setup controls for tracing.
+ */
+static struct trace_info *setup_controls(const char *root,
+ const char *name, u32 flags)
+{
+ struct trace_info *trace;
+ long ret;
+
+ trace = kzalloc(sizeof(*trace), GFP_KERNEL);
+ if (!trace) {
+ ret = -ENOMEM;
+ goto err;
+ }
+
+ trace->dir = create_tree(trace, root, name);
+ if (IS_ERR(trace->dir)) {
+ ret = PTR_ERR(trace->dir);
+ trace->dir = NULL;
+ goto err;
+ }
+
+ trace->state_file = debugfs_create_file("state", 0444, trace->dir,
+ trace, &state_fops);
+ if (IS_ERR(trace->state_file)) {
+ ret = PTR_ERR(trace->state_file);
+ trace->state_file = NULL;
+ goto err;
+ }
+
+ if (!flags & TRACE_FLIGHT_CHANNEL) {
+ trace->dropped_file = debugfs_create_file("dropped", 0444,
+ trace->dir, trace,
+ &dropped_fops);
+ if (IS_ERR(trace->dropped_file)) {
+ ret = PTR_ERR(trace->dropped_file);
+ trace->dropped_file = NULL;
+ goto err;
+ }
+ }
+
+ if (flags & TRACE_FLIGHT_CHANNEL) {
+ trace->reset_consumed_file = debugfs_create_file("rewind", 0444,
+ trace->dir, trace,
+ &reset_consumed_fops);
+ if (IS_ERR(trace->reset_consumed_file)) {
+ ret = PTR_ERR(trace->reset_consumed_file);
+ trace->reset_consumed_file = NULL;
+ goto err;
+ }
+ }
+
+ trace->nr_sub_file = debugfs_create_file("nr_sub", 0444,
+ trace->dir, trace,
+ &nr_sub_fops);
+ if (IS_ERR(trace->nr_sub_file)) {
+ ret = PTR_ERR(trace->nr_sub_file);
+ trace->nr_sub_file = NULL;
+ goto err;
+ }
+
+ trace->sub_size_file = debugfs_create_file("sub_size", 0444,
+ trace->dir, trace,
+ &sub_size_fops);
+ if (IS_ERR(trace->sub_size_file)) {
+ ret = PTR_ERR(trace->sub_size_file);
+ trace->sub_size_file = NULL;
+ goto err;
+ }
+
+ return trace;
+err:
+ if (trace) {
+ remove_controls(trace);
+ kfree(trace);
+ }
+
+ return ERR_PTR(ret);
+}
+
+static int trace_setup_channel(struct trace_info *trace, u32 buf_size,
+ u32 buf_nr, u32 flags)
+{
+ if (!buf_size || !buf_nr)
+ return -EINVAL;
+
+ if (flags & TRACE_GLOBAL_CHANNEL)
+ trace->rchan = relay_open("trace", trace->dir, buf_size,
+ buf_nr, &relay_callbacks_global,
+ trace);
+ else
+ trace->rchan = relay_open("trace", trace->dir, buf_size,
+ buf_nr, &relay_callbacks, trace);
+
+ if (!trace->rchan)
+ return -ENOMEM;
+
+ trace->flags = flags;
+ trace->state = TRACE_SETUP;
+
+ return 0;
+}
+
+/**
+ * trace_setup - create a new trace trace handle
+ * @root: The root directory name to place trace directories.
+ * @name: Trace directory name, created in @root
+ * @buf_size: size of the relay sub-buffers
+ * @buf_nr: number of relay sub-buffers
+ * @flags: Option selection (see trace channel flags definitions)
+ *
+ * returns a trace_info handle or NULL, if setup failed.
+ *
+ * The @root is created (if needed) in the root of the debugfs.
+ * The default values when flags=0 are: use per-CPU buffering,
+ * use non-overwrite mode. See Documentation/trace.txt for details.
+ */
+struct trace_info *trace_setup(const char *root, const char *name,
+ u32 buf_size, u32 buf_nr, u32 flags)
+{
+ struct trace_info *trace;
+
+ trace = setup_controls(root, name, flags);
+ if (IS_ERR(trace))
+ return trace;
+
+ trace->buf_size = buf_size;
+ trace->buf_nr = buf_nr;
+ trace->flags = flags;
+ mutex_init(&trace->state_mutex);
+ trace->state = TRACE_SETUP;
+
+ return trace;
+}
+EXPORT_SYMBOL_GPL(trace_setup);
+
+/**
+ * trace_start - start tracing
+ * @trace: trace handle to start.
+ *
+ * returns 0 if successful.
+ */
+int trace_start(struct trace_info *trace)
+{
+ /*
+ * For starting a trace, we can transition from a setup or stopped
+ * trace.
+ */
+ if (trace->state == TRACE_RUNNING)
+ return -EINVAL;
+
+ mutex_lock(&trace->state_mutex);
+ if (trace->state == TRACE_SETUP) {
+ int ret;
+
+ ret = trace_setup_channel(trace, trace->buf_size,
+ trace->buf_nr, trace->flags);
+ if (ret) {
+ mutex_unlock(&trace->state_mutex);
+ return ret;
+ }
+ }
+
+ trace->state = TRACE_RUNNING;
+ mutex_unlock(&trace->state_mutex);
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(trace_start);
+
+/**
+ * trace_stop - stop tracing
+ * @trace: trace handle to stop.
+ *
+ */
+int trace_stop(struct trace_info *trace)
+{
+ int ret = -EINVAL;
+
+ /*
+ * For stopping a trace, the state must be running
+ */
+ mutex_lock(&trace->state_mutex);
+ if (trace->state == TRACE_RUNNING) {
+ trace->state = TRACE_STOPPED;
+ /*
+ * wait for all cpus to see the change in
+ * state before continuing
+ */
+ synchronize_sched();
+ relay_flush(trace->rchan);
+ ret = 0;
+ }
+ mutex_unlock(&trace->state_mutex);
+ return ret;
+}
+EXPORT_SYMBOL_GPL(trace_stop);
+
+static void trace_cleanup_channel(struct trace_info *trace)
+{
+ trace_stop(trace);
+ relay_close(trace->rchan);
+ trace->rchan = NULL;
+}
+
+/**
+ * trace_cleanup - destroys the trace channel, control files and dir
+ * @trace: trace handle to cleanup
+ */
+void trace_cleanup(struct trace_info *trace)
+{
+ trace_cleanup_channel(trace);
+ remove_controls(trace);
+ kfree(trace);
+}
+EXPORT_SYMBOL_GPL(trace_cleanup);